Scratch-and-sniff smell test could help spot brain disease

22:54Sunday 15 February 200400:00Monday 16 February 2004

BRAIN diseases such as Alzheimer’s and the more mysterious psychiatric disorder of schizophrenia are difficult to diagnose in their earliest stages. Even now if you had access to the most sophisticated modern brain-scanning techniques, your chances of detecting the diagnosis early are remote. This is a pity because the earlier you can detect illnesses of this type and start treatment, the better the prognosis might be.

But now the latest medical research promises a breakthrough which might mean anyone, not just doctors, could spot the earliest signs of these serious illnesses using something as low-tech as a simple test of your ability to smell.

The kind of smell test increasingly being used by medical researchers includes a coated area on a test card which is scratched to release an odour including different microencapsulated common smells - for example, chocolate, pizza or grape. It takes just five minutes to administer the test during which the subject is required to scratch the odour strip and match what they have sniffed to one of four possible smells.

All of our senses, like touch, hearing and sight are brought to us through our brains, and often it is a defect in the function of these senses that first alerts us to a probable impairment in brain function. Of all the senses, the one that has been most neglected in medical research is the sense of smell - perhaps because unlike the rest of the animal kingdom, we rely on it less than our other senses to get us through the day.

For example, if you stretched out the surface area of the cells in your nose used to detect smell this would stretch to a surface area of 22cm squared.

This seems fairly remarkable until you understand that the same cells stretched out cover an area of seven square metres in the German shepherd dog.

But of all the human senses, olfaction, or smell, is also unique in that only one nerve cell junction, or synapse, lies between the receptor cells in your nose and the parts of the brain - the olfactory cortex - where the sense of smell is finally processed. This means the sense of smell is actually the most direct link between our brains and our environment.

The number of cells in the nose used to detect smell is more than for any other sensory system except vision.

And there is now accumulating recent evidence that one of the earliest and most reliable indicators that you could be about to develop a serious brain disorder, like schizophrenia or Alzheimer’s disease, could be early impairments in the ability to smell.

Olfaction is processed largely in the frontal and temporal lobes of the brain - the front and sides - which are those areas that have been implicated in schizophrenia. So it follows that maybe the earliest signs of the development of the disorder could be brain dysfunctions that also produce deficits in the sense of smell.

In particular, it is the frontal lobes of the brain - these are the areas implicated in functions like judgment and planning, where smell and schizophrenia are both located. This means that doctors might be able to exploit this coincidence to use your sense of smell to predict whether you are about to get schizophrenia or other disorders.

The very latest medical research has established that a deficiency in the sense of smell is not only seen very early in the course of schizophrenia, it is also strongly associated with the eventual duration of the illness. So using smell as an early test for schizophrenia cannot only predict the onset of the disease but also how severe it’s going to be. It seems the worse your inability to smell the longer you are going to suffer from schizophrenia.

Eighty per cent of schizophrenia patients exhibit deficits in odour identification which are severe enough to interfere with daily functioning, in contrast to less than 15 per cent of the general population.

Whether the powerful drugs that schizophrenics are usually taking when they are recruited into studies on smell could be the actual cause of the smell detection deficits has also been investigated.

Those schizophrenics not on any medication at all have been found to suffer from similar levels of smell detection problems, so medication cannot be the substantial cause.

Dr Bruce Turetsky a neuropsychiatrist at the University of Pennsylvania, has recently found that the nerve cells which detect smell that line the nasal passages are more immature when autopsies are performed on people with schizophrenia.

But why should the sense of smell be linked to schizophrenia? Controversy continues over whether the adult brain can regenerate nerve cells and grow new ones, yet it is intriguing to note that one of the very few areas in the brain where we know for certain that nerve cell growth continues in adulthood is the area that deals with smell.

No-one knows for sure why this is, but it does mean that any impairment in nerve cell development might manifest itself most acutely in this part of the brain and therefore in the sense of smell.

Another interesting theory is based on the idea that, of all the senses, that of smell is perhaps one of the most "emotional".

After all, a particular smell will frequently reawaken an especially strong emotional memory, plus the multi-billion-pound perfume industry largely trades on the ability of smells to induce certain moods.

The key then is that smell is intricately linked in the brain to those centres responsible for emotional response, and it is these areas which often appear defective in schizophrenia.

A group at the University of Pennsylvania led by neuroscientist Dr Paul Moberg has published new data which found that male schizophrenics are particularly poor at emotional responses to particular smells.

What is intriguing about this finding is that it suggests a possible test that could predict the future onset of schizophrenia, which is not just your ability to detect smells but a gauging of your emotional reaction to them.

It seems that it is not just whether you can smell or not that now interests your doctor, but whether you can emotionally respond to a smell in the way you could have several years ago. Perhaps in the future a yearly blood pressure check will be accompanied by a scratch-and-sniff smell test.

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